The invention relates to a process for producing cellulose triacetate films that are particularly suitable for use as supports of photographic materials. More specifically, the invention relates to a process for producing a cellulose triacetate film, which process includes steps of casting a solution of cellulose triacetate (herein referred to as a "dope") onto an endless band or drum (herein referred to collectively as a "support") having its surface cooled to 20.degree. C. or below, drying and cooling the cast solution on the support to solidify the same, separating the solidified film from the support, and thereafter transporting the film while it dries on transport rollers.
A conventional process for producing cellulose triacetate films by casting on a drum is depicted in FIG. 3(a). A cellulose triacetate film 3 cast onto a rotating drum 5 from a casting die 1 is dried to some extent on the drum 5, from which a film 4 is stripped and subsequently transported on transport rollers 7.sub.1, 7.sub.2, 7.sub.3, . . . while it is dried on both sides.
A conventional process for producing cellulose triacetate films by casting on an endless band is depicted in FIG. 3(b). A cellulose triacetate film 3 emerging from a casting die 1 is cast onto an endless band 6 wound onto two rotating drums and then dried to some extent on the band 6, from which a film 4 is stripped and subsequently transported on transport rollers 7.sub.1, 7.sub.2, 7.sub.3, 7.sub.4, . . . while it is dried on both sides.
For increasing the production rate of the processes described above, it is effective to maximize the proportion of residual solvents in the cellulose triacetate film 3 so that the film 4 can be separated from the support such as drum 5 or band 6. Many proposals have been advanced to implement this approach (see, for example, U.S. Pat. Nos. 2,221,019, 2,607,704, 2,739,069, Examined Japanese patent publications Nos. 9074/1970 and 48862/1979, as well as Unexamined Published Japanese patent application No. 115035/1987).
Stripping the cellulose triacetate film 3 from the support 5 or 6 to form the film 4 can be accomplished by the combination of the following two steps: evaporating the solvents by drying on the support 5 or 6, and cooling the support until its surface temperature drops to a level that is below the solidifying point as determined by the amount of residual solvents in the film at the point of separation from the support.
In practice, however, when the solution of cellulose triacetate emerging from the casting die 1 is cast as the film 3 onto the support 5 or 6, the solution causes a "neck-in"problem in the cast area (indicated by 2 in FIG. 2(a)), and, as shown partially enlarged in FIG. 2(b), the edge on either side of the cast area 2 will eventually collect on either side of the film 3 to increase its thickness. As shown in FIG. 1(b), the film 4 subsequently separated from the support will have an undesirably thick portion in area B on either side of the film where it is in contact with the transport roller 7 and which is about 30 mm wide.
After being separated from the support 5 or 6, the film 4 is rapidly dried on both sides. However, in the early stage of transport by rollers where the film retains a high proportion of residual solvents, the temperature of the film 4 at the edge on either side will increase due to the heat transferred from the nearest transport roller, and hence the film becomes viscous again (becomes a dope) in those areas and sticks to that transport roller to foul it.
This fouling of the transport roller is most likely to occur in area B (see FIG. 1) where the surface temperature of that roller is particularly high.
The fouling deposit on the transport roller will grow with time until it is dislodged from that roller and redeposited as foreign matter on the next roller. The resulting foreign matter can cause indentations and other surface defects that impair the appearance of the final film.
Conventionally, this problem has been dealt with manually by the operator who periodically cleans the transport rollers during production. The common method of cleaning the transport rollers is to manually wipe off the dirt on the rollers with a cloth wetted with an organic solvent such as methylene chloride or methanol. A disadvantage of this method is that the operator must take the risks associated with inhaling noxious gases from the solvent. As another problem, the film being transported during the cleaning operation cannot be processed to a final product while the cleaning operation is being performed, so that the efficiency of production will decrease considerably if the cleaning operation is performed frequently.